One way or another, they tend to overlap. Who makes up the NCAA? Colleges, universities athletic conferences and other affiliated organizations are NCAA members. These members help set up the rules. Who pays the coaches? The colleges do. Why do you suppose college athletes don't get paid? Who is the biggest beneficiary of college athletes not getting paid? Are we connecting any dots here yet? The colleges are collecting all the money. They dole out a small portion to the coach while keeping the rest. It's of little wonder why Nick Saban is being paid more money per year than the highest paid NFL coach (Pete Carroll). In fact, there are two college coaches paid more than the highest paid NFL coach. Could it be because in the NFL the money has to be distributed to the players?
When you say the players aren't being forced to do this, I'm not sure if you are serious. Of course the players are being forced to do this. How is a player ever to make a NFL roster unless they go through the motions of playing college ball? The NFL has already catered to the NCAA setting up a rule that a player has to be three years removed from high school before they are eligible to enter the draft. If they ever want to get paid to play, they are essentially forced to do this.